Parasite in Horses: A Primer

When you look out across your pasture to watch your horse grazing, you might only see the obvious--a 1,000-pound friend and teammate. But if you were to look beyond the surface, you'd discover that he is really an ecosystem unto himself.

Your horse's body (and that of every other horse) is host to thousands, perhaps millions, of microscopic creatures that use a share of his oxygen and nutrients, and enjoy the body heat he generates and the shelter of his internal recesses. At best, these hitchhikers live symbiotically, providing benefits of their own in exchange for what they take from him. The microbial populations that live in his cecum and help him digest plant fibers are a good example.

At worst, they are parasites--freeloading organisms that need their hosts to provide a living environment and nutrients while contributing nothing to the relationship, and usually leaving their "dwellings" in worse shape than how they found them.

To some extent, harboring parasites is a natural and normal condition for equines. In the wild, however, parasites rarely build up to the point of seriously damaging their hosts' health. (After all, it's not to a parasite's advantage to kill its host, as that would guarantee the death of the parasite as well!) Not only does a feral horse's wandering, grazing lifestyle limit the amount of parasites he picks up from his environment, but if he's repeatedly exposed to low levels of a parasitic invader, he usually develops some natural immunity.

Domestic horses, on the other hand, find parasites a potentially devastating threat to their health, largely because they are kept in confined spaces where the parasite load can build up to dangerous levels. Without intervention, parasite population explosions can result in an ulcerated and bleeding digestive system, recurrent colic, damaged and irritated lungs, or blood vessels damaged by developing worms.

Even at low concentrations, internal parasites have a less than ideal impact on your horse's health and well-being. Parasites steal nutrients from their host and can leave him undernourished and anemic. They can produce open sores and intense itching. In the respiratory system, they can trigger persistent coughing and nasal discharge. In young horses, they can be responsible for stunted growth and a tendency toward unthriftiness.

Of all the parasites that might be lurking in your horse's darkest internal corners, worms and bots are the ones with which horse owners are generally most concerned, perhaps because they're the most visible of the parasites. Many of us have seen evidence of their occupation as adult worms or bots in our horses' manure. And let's face it--the very idea of them wiggling around inside our beloved beasties grosses us out. That in itself might be enough to motivate us to want to eradicate them. But the potential damage of which they're capable is a much better reason to wage war.

Of course, having battles with Mother Nature is seldom a simple procedure. Worms and other internal parasites have proven fairly ingenious in avoiding extermination. In the last 50 years or so, we've come up with some impressively effective ways of killing worms without killing their hosts, but while we win many skirmishes, we will probably never win the war.

Parasitic organisms are such a pervasive part of the environment that, a century from now, horse owners will more than likely still be fighting them, without much more success than we currently enjoy.

That said, we have definitely gained some ground. Those pot-bellied, run-down, patchy-haired creatures that we know as the classic "wormy" horse are a rare sight these days. Researchers who study equine health attribute the increasing tendency of domestic horses to live longer mainly to the development of effective deworming medications. Deaths from parasitism have been reduced. So in many ways, we've reduced worms from a "deadly threat" to a "perpetual nuisance." But that's no reason to let our guard down.

You can't completely eliminate the impact of internal parasites on your horses. But if you want to weaken their ranks and keep their damaging effects to a minimum, it's helpful to know the enemy's habits and vulnerabilities. That way, you'll know how to get them where they live. Here's a primer on major equine parasites that will allow you and your veterinarian to map out a battle plan in the fight against them.

Know Thy Enemy

It's estimated that more than 150 species of internal parasites can use the horse as a suitable host. Of those, there are 10 categories of major importance. These are the invaders that affect the most horses world-wide, and which have the greatest potential to do harm.

Large Strongyles--Often called bloodworms or redworms, large strongyles are probably the most dangerous class of equine worms today. Even small populations can seriously affect a horse's development and performance, and large loads can cause emaciation (severe thinness), anemia (deficiency of red blood cells), and death due to aneurysms or clots in major arteries.

Strongyles are found wherever horses graze, so virtually all horses become infected early in life. No age-acquired immunity seems to develop, as it does with some other worm species, so all ages of horses are vulnerable to these invaders, and large worm burdens can accumulate easily.

There are three species of large strongyles that affect horses. Strongylus vulgaris, a worm that averages about 2 centimeters (cm or 0.8 inches) in length at maturity, is considered the most dangerous because its life cycle takes it through the mesenteric artery system (supplying the intestines), where it has the potential to trigger blood clots and extensive damage to the arterial walls, as well as interfere with the blood supply to the digestive tract. The eggs of this worm are found in horse manure. This warm and hospitable environment triggers the eggs to hatch, and the larvae crawl up onto grass blades where they're ingested by the next horse to come along. (Under cold and dry conditions, the larvae can survive for months at a time, ready to emerge when conditions are favorable.)

Once ingested, the larvae penetrate the intestinal wall and migrate to the blood vessels supplying the small and large intestines. There they grow and develop for three to four months before returning to the gut as young adults and lodging in the interior surfaces of the cecum and the colon. Around each worm, a nodule forms in the tissue, which eventually ruptures to release the mature worms into the large intestine. There they breed and release their eggs to be passed out in the manure and start the cycle anew.

The process, from egg to maturity, takes six to seven months. An adult female stron-gyle can produce up to 5,000 eggs per day for several months.

Strongylus edentatus--This is a larger worm that matures at 4-5 cm (1.6-2 inches) in length, and has a life cycle similar to Strongylus equinus (below) except that the larvae tend to migrate from the intestines to the liver via the portal vein. After a few weeks of wreaking havoc in liver tissues, it travels back to the abdominal peritoneum, then to the walls of the horse's cecum. There it forms large nodules where it burrows into the tissues. S. edentatus takes between 10 to 12 months to complete its life cycle. An adult female can produce several thousand eggs per day for several months.

Strongylus equinus--This is the parasite in this family we know the least about. This parasite is considered rare in managed horses. Its migratory pathway is only partially understood, although we know it migrates from the intestines to the liver via the peritoneal cavity and remains in the liver for six to seven weeks. Late-stage larvae are also found in the pancreas and adjacent organs, where they molt to become immature adults about nine months after infection. They complete their life cycle in the intestine like the rest of the strongyle species, although how they get back there is still unknown.

Among the effects of any strongyle infestation are weakness, loss of condition, diarrhea, and anemia. Large concentrations of larvae in the blood vessels can promote clot formation, which interrupts the normal blood flow and is potentially very dangerous. These parasites also can cause the arterial walls to weaken and balloon out (an aneurysm); these arteries sometimes can rupture, causing sudden death. The nodules formed by the adults in the intestinal wall can lead to bleeding and ulceration of the mucosal surfaces; the scarring from this can be extensive and can affect the horse's ability to absorb nutrients.

Three consecutive treatments at intervals of five months using ivermectin and moxidectin are recommended to eradicate large strongyles.

Small Strongyles--Also known as cyatho-stomes, these parasites for many years were considered less dangerous than the large strongyles, but their resistance to many common deworming drugs has made them the subject of increasing concern to researchers and veterinarians. They have a unique ability to remain encysted in the walls of the large intestine for months or years, impervious to most types of chemical assault. Fenbendazole, moxidectin, ivermectin, and pyrantel tartrate kill adult stages and larvae of small strongyles in the lumen of the large intestine (see "Small Strongyles" in the April 2000 issue of The Horse, article #221).

When masses of small strongyles emerge at one time from the gut wall in spring (after over-wintering there), they can trigger severe irritation, diarrhea, and recurrent colic.

There are about 40 species of small strongyles that commonly affect horses and other equids, varying in size from about a half-centimeter to 2 cm (0.2-0.8 inches) in length. Like the large strongyles, small strongyles hatch in manure and can be ingested by a horse, and the larvae then penetrate the mucosa (lining) of the cecum and colon. Unlike the large strongyles, small strongyles stay in the gut walls for their entire life cycle instead of migrating to other areas of the body.

Researchers don't yet fully understand why small strongyles sometimes emerge from the encysted stage en masse, but the accumulated metabolic wastes that are released with the worms when they're released seem to have a lot to do with the ensuing inflammation and digestive upset.

Threadworms--These also are known by the Latin name Strongyloides westeri and bear the dubious distinction of being the first parasites to invade the young foal. They can be passed through the mare's milk, and can be found in foals as young as four days old. The life cycle of the threadworm is an accelerated one--they mature in five to seven days--so by the time a foal is a couple of weeks old, he might already be shedding S. westeri eggs into the environment and providing the means to re-infect himself. A five-week-old foal's manure can contain up to 20,000 threadworm eggs per gram.

These worms are creative in their means of accessing a foal's system; they can even gain entrance by penetrating the skin when the animal lies down on infected bedding.

In the couple of weeks when a threadworm is maturing in a young horse's system, it migrates through the lungs and the small intestine, creating tissue damage as it goes and triggering diarrhea that is sometimes severe. Fortunately, threadworms aren't a lasting threat. A strong immunity generally develops by the time the foal is weaned, and these worms are rare in adult horses. Therefore, current recommendations are to deworm foals at two to four weeks of age, and give one or two more treatments at four-week intervals. Consult your veterinarian for specific recommendations on dewormers. Treating the mare in the last month of gestation can help prevent transmission of threadworms from the mare to the foal in the milk.

Ascarids--Also known as roundworms, Parascaris equorum parasites also tend to be more of a problem for youngsters; horses generally develop an immunity by the time they are two years old. Roundworms are large, measuring 15 to 55 cm (6-22 inches) long, and can number in the hundreds in the small intestine. They're particularly good at robbing a young horse of essential nutrients, leaving him looking pot-bellied, thin, rough-coated, and unthrifty. He might develop diarrhea, colic, respiratory disease, a nasal discharge, and a persistent cough that doesn't respond to antibiotics. If a heavy burden of roundworms develops in a youngster, he can suffer an intestinal blockage, which could be fatal unless surgery is performed to remove the mass of worms.

Foals acquire roundworms by ingesting eggs in contaminated feed or water. (The eggs can remain viable in soil for years.) The eggs hatch in the small intestine, and the larvae burrow through the intestinal wall, migrating to the blood vessels and taking a convoluted journey through the liver, heart, and into the air sacs of the lungs over a period of about a week. During that stage, they can create substantial irritation and scarring in the liver and the respiratory tract. From the lungs, the worms travel up the trachea to the mouth, only to be swallowed a second time. They mature in the intestines in two to three months, then lay eggs that are passed in the manure. A female ascarid can lay up to 200,000 eggs per day.

There is some danger in treating foals infected with a large burden of roundworms with highly effective dewormers. A massive killing of the numerous parasites can produce intestinal blockage, or trigger a toxic response to the large quantity of dead and dying worms in the digestive tract. For that reason, veterinarians often recommend using a lower dosage of the benzimidazole fenbendazole followed by a second treatment 10 to 14 days later with a more effective regumen.

Pinworms--These parasites are technically known as Oxyuris equi. They have pointed tails that make them vaguely resemble a pin. The females are much larger than the males, sometimes reaching 10 cm (4 inches) in length. If you see a young horse frantically rubbing his hindquarters against a wall or fencepost, chances are that pin-worms are to blame. The females lay their eggs around the anus (they're often visible as yellowish, gelatinous streaks on the perineal skin), where they cause intense itching.

The pinworm life cycle is relatively simple. Horses ingest the eggs from the soil or surfaces where an infected horse has rubbed. The eggs hatch in the small intestine, then pass into the colon and cecum, where they feed on the intestinal mucosa and mature over a period of about five months. The females pass out in the manure after laying their eggs around the anus; they can often be seen as whitish threads in the fecal matter.

Although pinworms cause some irritation of the tissues in the digestive tract, they're considered the least harmful of the equine internal parasites; the most serious damage from a pinworm infestation comes from the horse scratching himself until his tailhead becomes raw, inflamed, and bald.

Pinworm eggs can't tolerate high temperatures, so harrowing pastures in mid-summer is often helpful in decreasing pasture infestation. Dewormers such as ivermectin, benzimidazoles, moxidectin, and pyrantel pamoate are available and will remove these parasites.

Tapeworms--For many years tapeworms weren't considered a major equine problem, but that outlook is now being challenged. Two species, Anoplocephala magna and Anoplocephala perfoliata, are commonly found in clusters around the ileocecal valve in the cecum, where they can interfere with the digestion of nutrients. Occasionally they can accumulate to the point of causing cecal ulcerations, perforations, intussusceptions (slipping of one part of the intestine into another, usually blocking it), colic, and peritonitis. In North America, it's estimated that the frequency of infection averages up to 50%, but tapeworms are a much more com-mon pest in some other parts of the world. In Australia, for example, it's estimated that up to 62% of horses harbor tapeworms.

The tapeworm life cycle, unlike that of many other worms, requires an intermediate host--a particular kind of mite that lives in pasture grasses. The infection process begins after a horse ingests the mites. In two to four months the tapeworm matures, often reach-ing astonishing lengths of up to several feet. Unique among equine parasites, it is a segmented worm, and might lose individual segments from its tail end without losing its grip on the intestinal wall.

Tapeworms belong to the only worm family unaffected by ivermectin and most other deworming drugs. In fact, there are currently no drugs with a specific label claim for efficacy against equine tapeworms, but most veterinarians recommend a double dose of pyrantel pamoate (Strongid) to rid your horse of these worms.

Stomach Worms--These are one of the lesser-known categories of equine internal parasites, but they're actually found through-out North America. The three species include Habronema muscae, Draschia megastoma, and Trichostrongylus axei, but Habro-nema muscae is the only one that can be considered common.

Stable flies and house flies serve as the intermediate hosts for Draschia and Habronema by infecting the horse when they lay eggs on or near skin abrasions and small wounds. The flies can pass the parasites through their mouth parts when they are feeding around the lips of the horse. Larvae that hatch in wound sites don't mature, but they do linger, irritating the tissues to create persistent "summer sores," which are extremely itchy and resist healing. The sores often close in the winter months only to reappear when the weather warms up again.

Stomach worms that are ingested develop to maturity in the stomach, causing gastritis, loss of appetite, and sometimes hemorrhage. In addition, adult Draschia worms provoke the formation of large, tumor-like growths in the stomach, which can lead to fatal peritonitis.

T. axei is a tiny, non-migratory parasite that matures in the stomach and/or small intestine over the course of about 25 days. It's an equal-opportunity parasite, infecting pigs and cattle as well as horses. Heavy infestations lead to diarrhea and gastritis, and can interfere with nutrient absorption. Ivermectin is effective against this type of worm.

Filarids--From the Onchocerca species, the adult worm lives deep in the tissues of the neck. They produce larvae that travel through the skin and cause intensely itchy skin lesions, thinning hair, and scaly patches on the neck, head, chest, and belly. They occasionally invade equine eyes and cause lesions. The infections are often cumulative, and unless treated can reach a peak by the time a horse is six years old and stay at high levels throughout the rest of his life. Fortunately, these parasites usually respond to ivermectin; the drug is the main reason why Onchocerca infestations are far less common today than they were 20 years ago.

Lungworms--Dictyocaulus arnfieldi are slender, thread-like parasites primarily found in donkeys and mules, although horses which share a stable or paddock with these long-eared cousins can become infected. Donkeys usually don't show clinical signs of infection, but can pass larvae in the feces.

The donkey ingests larvae while grazing; the larvae then move through the intestinal wall and are carried to the lung, where they develop into adult worms up to 10 cm (4 inches) in length. The process takes two to four months, after which time eggs are laid. The eggs are carried to the throat by normal mucus production, then are swallowed and passed in the feces. Once deposited on the ground, they quickly hatch into larvae which, when ingested, begin the whole infective process again.

Lungworms cause persistent coughing and lung irritation in the horse, occasionally leading to bronchitis and pneumonia due to aspiration of the eggs and larvae into the alveoli of the lungs. Heavy infections might progress to emphysema. Ivermectin is effective against lungworms.

Bots--These equine parasites aren't really worms; they're the larvae of the bot fly, a bumble-bee imitator that lays its characteristically sticky yellow eggs on the leg and/or muzzle hairs of the horse in mid- to late summer. When horses lick the regions where the eggs are clustered, the eggs are stimulated to hatch. The larvae immediately burrow into the mucosa of the tongue and gums and migrate to the horse's stomach over a period of three to four weeks. They set up residence by attaching to the stomach lining, where they overwinter, eventually growing to a size of about 2.5 cm (one inch) long apiece.

In the spring, the fully developed larvae release their hold on the stomach, pass out in the manure, and burrow into the ground, where they pupate. In two to three weeks, the adult flies emerge, and the cycle starts anew.

There are three main species of bot fly, all of the genus Gasterophilus; the main difference in their life cycles is where they lay their eggs. The common bot fly, G. intestinalis, concentrates on the hairs of the horse's forelegs, belly, flanks, and shoulders. The nose bot fly, G. haemorrhoidalis, lays black eggs on the hairs of the lips. The throat bot fly G. nasalis attaches its yellow eggs to the hairs of the throat.

Bots usually have no serious inmpact on your horse's digestive health. The egg-laying activity of the adult flies is more irritating than anything the larvae do inside the stomach.

Bot flies are so pervasive in North America that it's safe to assume almost all horses are infected. Since the adult flies are killed by a hard frost, treating your horse for bots in late autumn will minimize re-infection until the following year. Many parasitologists recommend administering a medication effective for bots every 30 to 60 days during the spring and summer months to minimize the damage the larvae do to the stomach lining and the tissues of the mouth.

Our Side Of The Battle

A hundred years ago, there wasn't much in the average veterinarian's arsenal that was effective against parasitic infections. Most of the treatments were based on folklore rather than fact, or were so toxic to the horse that the cure was likely to be worse than the disease. In the 1800s, for example, tobacco juice and opium were two common "de-wormers;" neither had much effect on the parasites.

After World War I, carbon disulfide became a popular dewormer, and it did have some effect. But because it had to be administered by stomach tube, there were a high frequency of adverse reactions (usually through respiratory failure and/or coma). This method is no longer used.

The 1950s saw the introduction of the first modern equine parasiticide, phenothiazine. This was highly effective against large and small strongyles, but had limited efficacy against bots and roundworms. Small strongyles developed resistence to phenothiazine by the early 1960s. For the next 10 years, phenothiazine continued to be marketed in combinations with piperazine and/or an organophosphate to compensate for these deficiencies.

The early 1960s saw the introduction of organophosphate medications such as trichlorfon and dichlorvos. These drugs had a somewhat higher safety margin than carbon disulfide (larger difference between the recommended dose amount and the toxic overdose amount), and were effective against bots as well as having some action against other types of parasites. However, they weren't necessarily recommended for young foals or pregnant mares.

In the 1970s, pharmaceutical companies began marketing a new class of dewormers. The benzimidazoles were safer than organo-phosphates and had better efficacy against strongyles, roundworms, and pinworms, but weren't particularly useful against bots. They were often combined with organophosphates. These combinations provided a one-two punch that offered the horse owner something approaching truly effective parasite control for the first time.

The introduction of ivermectin truly revolutionized the job of parasite control. Some 20 years after it was first made available, it is still popular, with unrivaled efficacy against almost all classes of internal parasites except tapeworms. Ivermectin also has the ability to kill external parasites such as mites and Habronema larvae in summer sores, and it has a very wide safety margin, making it a useful over-the-counter medication. Recently, it was joined on the market by moxidectin, a drug in the same "family" that has similar efficacy, but also has a unique ability to control small strongyles in their encysted phase in the gut wall. (Recent studies have indicated that twice the clinical dose of fenbendazole administered for five consecutive days is also effective against encysted small strongyles.)

With drugs like these available in easy-to-administer oral paste or gel forms, deworming by nasogastric tube has practically become a thing of the past, and controlling parasites has mostly passed from the hands of veterinarians to horse owners.

However, this revolution hasn't been without its problems.

Dealing With Dewormer Resistance

Drug resistance is the biggest obstacle in our continuing war on worms, and the ease with which we can now administer de-worming medications might be escalating the resistance problem. This is because for many of us, it's easier to dose our horses frequently and indiscriminately with a tube of paste dewormer than it is to have fecal analyses performed to tell us which worms the horse has.

As with antibiotics, the more often we expose a pathogen to a medication, the more opportunities we give it to find a way to avoid or resist its effects. Resistance frequently outstrips the rate at which we can develop new drugs, and this is certainly true in the case of deworming medications. Moxidectin is the only significantly new drug that has been introduced in the past two decades.

Andrew Peregrine, BVMS, PhD, DVM (UK), Associate Professor in the Department of Pathobiology at the Ontario Veterinary College, University of Guelph, says the horse industry has been very lucky when it comes to deworming drugs because so far, as problems with resistance have been relatively limited. But to assume this state of affairs will continue indefinitely "is being a little naï¿½ve," he says. "It's coming--hopefully not in our lifetime, but it's coming. Parasites are very successful at adapting."

Researchers have already noted widespread resistance to the benzimidazoles, and to a lesser extent a resistance to pyrantel.

In other species, most notably cattle and sheep, problems with resistance to deworming drugs, including ivermectin, are rampant. In some parts of Australia, for example, resistance is so widespread that some farmers have had to stop raising sheep. Parasites of sheep have a much shorter life cycle than most equine ones, might cause outright fatality, and farmers tend to deworm sheep far more aggressively, and frequently, than horse owners do. But the resistance seen in that species still serves as an important warning sign.

"We should be doing all we can to preserve the power of our current de-worming medications," says Peregrine.

"If you use different compounds each time you deworm, you have the highest chance of resistance," Peregrine says. "You expose each generation of parasites to a wide variety of different drugs."

A better solution, he suggests, is to rotate deworming products once a year. For example, use ivermectin for one year, then switch to pyrantel the next. When you do this, use the same drug on all horses on your farm. "Make sure you switch drug classes each year," he emphasizes. "Be careful you don't switch from one benzi-midazole to another, for example."

It's also important to resist the temptation to overuse dewormers. A horse's natural immunity can be a powerful factor in his ability to fight off parasite infestations, but in order to develop that immunity, he has to be exposed to the parasites. Although owners might be horrified at the thought, carrying a low level of worms is a natural state for wild equines, and most suffer few ill effects. It's only when we crowd domestic horses on small pastures, multiplying the exposure to and thus the effects of parasitism, that extreme health problems develop.

So, while we'd like to imagine that our horses are always parasite-free, the reality is that having a few internal hitchhikers might do them no real harm. You actually could be doing your horse a favor by letting his system deal with the parasites to a certain degree. That's not to say you shouldn't employ deworming drugs, just that you should use them judiciously.

According to Peregrine, adult horses who've developed a healthy immunity of their own should only be dewormed, according to the manufacturer's directions, for the first four months they are on new pasture. This "strategic deworming" takes into account the natural life cycle of the worms and attacks them when they are most vulnerable, he says.

Fecal checks can become a valuable part of your parasite control program. The presence of eggs, larvae, and/or adult worms in your horse's manure can give your veterinarian a much better idea of the level and type(s) of parasites the horse harbors, and lead to the best treatment schedule.

"I recommend taking fecals from as many horses as possible every year or two, in the summer months, on the day of treatment (with a dewormer) and again 14 days later," says Peregrine. "You should see a 90% reduction in egg counts (eggs per gram of feces)."

To get a fecal analysis, gather two or three fresh manure balls in a sealed bag and take them to your veterinarian for testing.

"Targeted" deworming, in which only horses with high egg counts get dewormed, might be the best strategy for warding off drug resistance. "But if you do it, you've got to do it seriously," Peregrine says. "It takes diligence, and frequent fecal checks, and most people would rather just buy a tube of de-wormer, administer it, and hope for the best."

In the case of daily dewormers, Peregrine says they work fine, "and the incidence of colic decreases significantly for some horses, which is probably an indication that it's cleaning out the large strongyles. But because you're continuously exposing the parasite to low levels of the drug, you do risk resistance."

Peregrine says that this also raises the question of whether daily dewormers prevent your horse from developing natural immunity. If that is true, then once removed from the daily dewormer regimen, it's possible that the horse could be more vulnerable than usual to a parasite onslaught (initially, at least).

Dosing appropriately is also important when you're trying to wipe out parasites without giving them the chance to develop resistance. Paste, gel, and liquid formulations are designed to be administered according to the weight of your horse (you can easily get a rough estimate of this using a heart-girth weight tape, available for a couple of dollars from your feed store). Too low a dosage might provide an incomplete kill and allow the parasites to bounce back (although there are instances where that effect is appropriate, such as when you suspect a heavy roundworm infestation in a foal and want to avoid a massive die-off, as described earlier). Most modern dewormers have a high safety margin. "It varies from drug to drug, so it's important to read the package directions," says Peregrine. "With ivermectin, for example, you'd have to give 10 to 20 times the normal dose to get any toxicity. Panacur's toxicity level is something like 5,000 times the therapeutic dose. But there have been a couple of cases of death due to overdose with moxidectin, particularly with foals. It's safe up to about four or five times the recommended dosage."

Manure Management And Other Strategies For Control

Because manure contaminated with parasite eggs is the most common way in which worms re-infect your horse's environment, managing the amount of manure to which your horse is exposed is a useful way of reducing, if not eliminating, his parasite load.

Start by situating your manure pile far from grazing areas and water sources, and follow composting procedures so that the interior heat of the pile destroys parasites (See "Manure Management in the October 1997 issue of The Horse, article #641). Muck your stalls on a daily basis so that your horse isn't exposed to roundworm and pinworm eggs when he's in a confined area.

Removing manure from your pastures twice a week is the most effective way of reducing your horse's parasite load. If you have considerable acreage, however, this is impossible. If you can't remove the manure, harrow the land to break up the manure piles and expose the eggs to the elements, which will destroy some of them provided you do it only in hot, dry weather or when it's below freezing (but snow-free). Moist, temperate conditions encour-age eggs to hatch and larvae to thrive, so if you harrow in the spring or fall you'll likely just spread and increase the parasite burden on your fields. After harrowing, allow the pasture to stand empty for at least two weeks before re-introducing your horses.

Avoid overcrowding your pastures, which can lead to an increase in the concentration of shed parasites.

It might be useful to allow your horses to share grazing space with cattle or sheep, if you have them, because with the exception of T. axei, there's little cross-transmission of internal parasites between species.

When you feed your horse, try to keep his hay and grain off the ground, or at the very least well away from manure deposits. Clean and disinfect buckets and water troughs periodically to ensure they're not contaminated by fecal material.

Isolate any new horses from the rest of your herd until you've had a chance to deworm them. Otherwise, try to deworm all of your horses at once so that they have less chance of re-infecting each other.

Design your deworming program according to the region in which you live. In climates where temperatures stay consistently below freezing in winter, your horse likely won't pick up new parasites until he's on spring pasture. (Not all types of worms are vulnerable to the cold, however; strongyles tend to survive it fairly well.) In the southern United States, by contrast, hot mid-summer temperatures will destroy worm eggs and larvae, so the best time to deworm horses there is in the autumn, winter, and spring.

If you're not sure of the situation in your area, consult with your veterinarian. Plan to do fecal egg counts on all of your horses to get a better idea of which parasites you're battling.

In general, you should direct parasite control against threadworms in the first six months and roundworms within the first year of your horse's life. Thereafter, the focus is on eliminating large and small strongyles, which are the most damaging equine parasites of adult horses. Ivermectin or moxidectin should be the keystones of your deworming program because of their broad-spectrum action and the (as yet) low incidence of resistance. Plan to rotate their use on a yearly basis with another class of drug, such as pyrantel or one of the benzimidazoles.

New Developments

Research into newer and better ways to eliminate equine parasites is proceeding "at a snail's pace," according to Peregrine. But there are a couple of treatments on the horizon that could become valuable additions to our anti-worm arsenal in the future.

"The newest thing is an increased interest in controlling tapeworms," says Tom Klei, PhD, a Boyd Professor and Associate Dean of Research at the School of Veterinary Medicine at Louisiana State University. "There's some argument over how important tapeworms are in the grand scheme of things--but some recent studies indicate that there's a correlation between certain colics and the concentration of tapeworms clustered at the ileocecal junction (where the small and large intestines meet). And of course they leave horrendous lesions (where they attach to the intestinal wall).

"There's a new product called Equimax," Klei says, "which is available in Australia, New Zealand, South Africa, South America, and Mexico. It combines ivermectin with praziquantel, an anti-tapeworm drug that has been used in dogs, cats, and other animals, even humans, but until now hadn't been used in horses. The combination of drugs means it's effective against every type of nematode horses are likely to get, as well as bots."

Unfortunately, Equimax is not yet available in the United States or Canada, and Klei is not sure when it might be.

Large and small strongyles are the focus of a completely different research approach, which uses a fungus to attack strongyle eggs in manure and on pastures. The fungus, which has been found to be non-toxic to livestock, could conceivably be packaged in a daily feed supplement. It would pass through the horse's digestive system untouched, according to Klei, and go to work killing parasite eggs passed in the horse's manure.

"It works, but how well we're not sure at this point," he says. "Probably the best application for it would be to help reduce our reliance on deworming drugs. An owner could dose her horses with moxidectin, for example, then feed this fungus a few weeks later, before eggs would be likely to be re-appearing in the manure."

No manufacturer has been found to com-mercially produce the fungus yet, but it might be something to watch for in the future.

Overall, the parasite picture has changed significantly since the introduction of ivermectin, Klei says. "We no longer see a lot of the problems with summer sores or microfilariae; those used to be common. And we no longer see the same incidence of colic caused by heavy loads of strongyles or ascarids. Those things have become a thing of the past.

"The danger with that," notes Klei, "is that people may think these parasites have been eliminated. They haven't; they're still in the environment. And if you stop using dewormers, they'll be right back in there."

With effective (at least for the present) drugs at our disposal, veterinarians and researchers are asking slightly different questions now, Klei says. Rather than dealing with major parasite crises, we need to examine the possible impact of lower levels of infestation.

"What does a sub-clinical infection of small strongyles do to the function of the gastrointestinal tract?" he asks. "We don't know yet. What level of control do you really need to maintain a horse's health? And why hasn't resistance to ivermectin appeared in horses to the extent that it has appeared in cattle and sheep? These are some of the things we're looking at."

Can Herbs Help?

With herbal supplements for horses growing in popularity all over the world, it should be no surprise that enterprising herbalists are now promoting herbal preparations as an "all-natural" alternative to deworming drugs. The advantage, manufacturers say, is a gentler anti-parasitic action, with no potential toxicity to the horse.

A quick survey of the Internet reveals more than 20 different herbs that are purported to have some action against internal parasites, including cayenne, goldenseal, wormwood (so named because it has been reputed for centuries to kill worms), and the highly toxic (which can cause severe founder) black walnut. The use of tobacco as a dewormer has also enjoyed something of a folksy renaissance, as has diatomaceous earth, a silica-rich powder that theoretically scours the parasites loose from their moorings.

But do any of these preparations work? So far, the evidence is next to nonexistent, says Tom Klei, PhD, a Boyd Professor and Associate Dean of Research at the School of Veterinary Medicine at Louisiana State University. "I'm not aware of any herbs which have significant activity against internal parasites. And there's clear data in several species which demonstrates that diatomaceous earth doesn't work."

The conclusion? "I wouldn't recommend using an herbal preparation. There's no evidence that they do anything, and your horse could suffer considerable damage from carrying a major parasite load, or from a herb which is toxic to his system. Deworming drugs are so safe and so inexpensive that there's no reason not to use them," Klei says.

* Broad spectrum anthelminticsï¿½Any compound that demonstrates efficacy against four distinct groups of worms: small strongyles, large strongyles, ascarids, and pinworms.
** Effective compoundsï¿½pyrantel pamoate (Strongid paste) at double dosage; pyrantel tartrate (Strongid-C) at the usual dose for 30 days; and praziquantel (which has been approved in the U.S. for treating tapeworms in dogs and cats).

About the Author

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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